Focused ion beam (FIB) systems are used in a variety of applications in integrated circuit manufacturing and nanotechnology to create and alter microscopic and nanoscopic structures. FIB systems can use a variety of sources to produce ions. A liquid metal ion source (LMIS), for example, can provide high resolution processing, that is, a small spot size, but typically produces a low beam current.
A typical system using a gallium LMIS can provide five to seven nanometers of lateral resolution. Such systems are widely used in the characterization and treatment of materials on microscopic to nanoscopic scales. A gallium LMIS comprises a pointed needle coated with a layer of gallium. The needle is maintained at a high temperature while an electric field is applied to the liquid gallium to extract ions from the source.
A FIB system with a gallium LMIS can be used, for example, to image, mill, deposit, and analyze with great precision. Milling or micromachining involves the removal of bulk material at or near the surface. Milling can be performed without an etch-assisting gas, in a momentum transfer process called sputtering, or using an etch-assisting gas, in a process referred to as chemically-assisted ion beam etching. U.S. Pat. No. 5,188,705, which is assigned to the assignee of the present invention, describes a chemically-assisted ion beam etching process. In chemically-assisted ion beam etching, an etch-enhancing gas reacts in the presence of the ion beam to combine with the surface material to form volatile compounds. In FIB deposition, a precursor gas, such as an organometallic compound, decomposes in the presence of the ion beam to deposit material onto the target surface.
In all of the processes described above, the function of the gallium ions in the beam is to provide energy, either to physically displace particles on the work piece by sputtering or to activate a chemical reaction of a molecule adhered to the surface. The gallium itself does not typically participate in the reaction. Gallium is used in the beam because its properties, such as melting point, ionization energy, and mass, make it suitable to form into a narrow beam to interact with commonly used work piece materials. There are disadvantages to using a gallium LMIS. Gallium atoms implant into the work piece and, in many applications, produce undesirable side effects, such as changing the opacity or electrical properties of a work piece. Gallium can also disrupt the crystal structure in the area of bombardment. Also, to produce a very narrow beam, the current in a beam from an LMIS must be kept relatively low, which means low etch rates and longer processing times.
While it would be desirable to use different ion species for different applications, liquid metal ion sources are limited in the type of ions they can produce. Only metal species that have suitable melting point, ionization energy, and mass can be used, and this limits the ion species available from an LMIS. While it would also be desirable to be able to rapidly change ion species for different processing steps, changing the ion species of an LMIS requires removing the source from the vacuum chamber and replacing it with a different source, which must then undergo a time consuming preparation procedure. There are liquid metal alloy sources that provide ions of more than one type of metal. The types of ions available from such sources are limited to combination of metals that form alloys having suitable properties. Some liquid metal alloy systems, such as the one described in U.S. Pat. No. 5,165,954 to Parker, et al. for “Method for Repairing Semiconductor Masks & Reticles,” are used with a mass filter that separates the ions of different species so that the beam impacting the target comprises a single species.
Some types of mass filters, such as an ExB filter or “Wien filter,” use an electric field and a magnetic field, perpendicular to the electric field, that pass ions of the selected mass and energy through the filter, while ions having other masses, or energies, are deflected into a barrier. The deflection of an ExB filter depends on the energy of the ions, and there is always some energy variation in the beam, so the filter introduces chromatic aberration into the system, spreading the beam and reducing its resolution. Non-uniform fields within the mass filter also contribute to beam aberration.
Plasma ion sources ionize gas in a plasma chamber and extract ions to form a beam that is focused on a work piece. Plasma ion sources, such as a duoplasmatron plasma ion source described by Coath and Long, “A High-Brightness Duoplasmatron Ion Source Microprobe Secondary Ion Mass Spectroscopy,” Rev. Sci. Instruments 66(2), p. 1018 (1995), have been used as ion sources for ion beam systems, particularly for applications in mass spectroscopy and ion implantation. Because of the energy spread of ions extracted from the plasma chamber, the ions of a duoplasmatron source cannot be focused to as small a spot as the ions from an LMIS. Duoplasmatron ion sources are used, for example, to implant ions over a large area or for time-of-flight mass spectroscopy. In time-of-flight mass spectroscopy, ions in the primary beam sputter ions from the surface, and the mass of each sputtered ions is determined by the time required for the sputtered ion to reach the detector. To obtain a precise measurement, it is necessary to known precisely when the ion in the primary beam impacts the surface. Many gases species consist of multiple isotopes having slightly different masses and because different isotopes in the primary beam will reach the specimen at different times, mass filtering is used to separate isotopes, so that a single isotope reaches the specimen at a precisely known time.
Recently, inductively coupled plasma (ICP) ion sources have begun to be used in FIB systems. Innovations in ICP sources have reduced chromatic aberration, allowing for higher resolution processing, which opens new opportunities for ion beam processing, including imaging.
Many different types of gases can be used in a plasma ion source to provide a variety of ions species, so the ion species can be optimized for different applications. For example, whereas helium ions are useful for imaging or light polishing, xenon ions provide higher milling rates that are useful for bulk processing. Plasma ion sources can produce ions of many different species and at larger currents, but beam resolution has been limited. When a user wants to change ion species in a plasma source, it is necessary to remove a first gas from the plasma chamber and replace it with a second gas. U.S. Pat. Pub. No. 2009/0309018 for “Multi-Source Plasma Focused Ion Beam System,” which is assigned to the assignee of the present invention and is hereby incorporated by reference, describes a system for providing multiple gases to the plasma chamber to provide different ion species for performing different charged particle beam operations. Unfortunately, it can take up to 30 minutes to remove one gas from the plasma chamber and fill it with a second gas. A gas inlet for a plasma ion source typically has a small opening through which gas is supplied to maintain the pressure in the plasma chamber. Because the gas is used very slowly, the small opening to replenish the gas is very small. This makes for an unacceptably long time to change out the gas for many applications that process a work piece sequentially using different process gases.
FIG. 1 shows a typical prior art ICP ion source 100 for use with a FIB system such as the one described in U.S. Pat. Pub. No. 2009/0309018. Gas is provided to a plasma chamber 102 within a source tube 103 from an external gas feed line 104 through a gas filter 106 and then to a capillary tube 108 with a flow restriction 110. Energy is fed into the plasma chamber 102 from RF power supply 113 by antenna coils 114 and ions are extracted through a source electrode aperture 116 in a source electrode 118 by extractor electrode 120. A split Faraday shield 121 reduces the capacitive coupling between the coil 114 and the plasma in chamber 102, in chamber 102 which reduces the energy spread of the extracted ions. Power supply 113 preferably drives the antenna 114 in a “balanced” manner, that is, the electrical phase shift across the antenna is adjusted to reduce modulation of the plasma potential as described in U.S. Pat. Pub. No. 20080017319 of Keller et al. for a “Magnetically enhanced, inductively coupled plasma source for a focused ion beam system,” which is assigned to the assignee of the present invention and which is hereby incorporated by references. The balanced antenna preferably provides a null point in the radio frequency energy field within the plasma, which reduces the energy spread of the ions extracted from plasma chamber 102.
The gas conductance into and out of the plasma chamber 102 is through the flow restriction 110 in the capillary tube (at the top of the source tube 103) and the aperture 116 (typically less than ¼ mm in diameter) in the source electrode 118. Pump 122 connected to gas supply line 104 through valve 123 removes gas from plasma chamber 102 through capillary 108 and gas supply line 104. An ion column pump (not shown) extracts gas from plasma chamber 102 through source electrode aperture 116. Multiple gas sources such as gas storage 130A, gas storage 130B, gas storage 130C and gas storage 130D supply gas into gas supply line 104 through corresponding valves 131A through 131D. A beam voltage supply 132 supplies a high voltage to the plasma in chamber 102 and an extraction voltage supply 134 supplies a voltage to extraction electrode 120. Extracted ions or electrons are focused by focusing electrode 136. Additional details of the focusing column and sample chamber are not shown.
To remove a gas from the interior of the plasma chamber, the gas feed line 104 is pumped as shown to remove gas in the source tube above the flow restriction 110 in the capillary tube 108. The volume of the FIB system below the source electrode 118 may also be adequately pumped using the main chamber vacuum pump(s) (not shown).
Because both the source electrode aperture 116 and the flow restrictor 110 have small diameters and correspondingly very low gas conductances, it is impossible to rapidly pump out the interior of the source tube 103. This is a disadvantage, particularly for a production FIB system where it is sometimes desirable to perform sequential process steps with different ion species. First, it may take a much longer time to pump out a first process gas from the source tube 103 before the base pressure is low enough to introduce a second process gas. Insufficient purging of the gas can lead to contamination of the plasma through ionization. U.S. patent application Ser. No. 13/182,187 for “Methods and Structures for Rapid Switching between Different Process Gases in an Inductively-Coupled Plasma (ICP) Ion Source” describes plasma chamber designs that provide for rapidly changing gas in a plasma source by providing an alternate path for gas to enter or leave the vacuum chamber.
Thus, providing high resolution beams of different ion species is limited by long gas exchange time or, in the case of a metallic alloy source, the metals present in the alloy which are typically limited by the ability to create such an alloy based on material compatibility.